The invention relates, generally, to cryogenic fluid storage and delivery systems and, more particularly, to an improved economizer system for horizontal storage tanks.
The typical cryogenic delivery and storage system is illustrated in FIG. 1 and includes a relatively large, vertically disposed storage tank 2. The typical storage tank is three to four feet tall and includes a double-walled, thermally insulated construction that holds a supply of liquid cryogen 4. As will be appreciated, even though tank 2 is insulated, heat leak will occur such that the liquid cryogen will slowly vaporize to create a gas head 6.
To deliver product from the tank 2 a dip tube 8 extends into the liquid 4 and is connected to a gas use device via a withdrawal line 10. A vaporizer 12 located in the withdrawal line 10 heats the liquid and vaporizes it before it is delivered to the use device.
Because heat is continuously transferred to the liquid in the tank, a portion of the liquid will vaporize thereby increasing the pressure in the tank. Where the tank stands idle for an extended period, the pressure rise in the tank can be relatively great. As will be appreciated, the pressure in tank 2 must be released to minimize the possibility of the tank or the associated plumbing from bursting or otherwise being damaged. One method to relieve pressure is simply to vent the gas to atmosphere; however, this procedure is wasteful and can be hazardous.
As an alternative to venting to atmosphere, an economizer circuit 14 has been developed as illustrated in FIG. 1. The economizer circuit 14 includes a line 16 connecting the gas head 6 to the withdrawal line 10. A regulator 18 located in line 16 will allow vapor to be delivered to the use device from gas head 6 when the pressure in the tank rises above the predetermined level set at regulator 18. By pulling vapor from gas head 6, instead of liquid through the dip tube 8, the pressure in tank 2 falls dramatically.
In the vertical tanks of the prior art (where the dip tube 8 is relatively long) gas is delivered via economizer line 16 whenever regulator 18 is open because the pressure head created by pulling liquid up the long dip tube makes gas flow out of the economizer circuit the preferred path. In other words when regulator 18 is opened and a demand for product is made, the path of least resistance is through economizer line 16 because of the pressure head of dip tube 8. Specifically, in the typical 3 to 5 foot tall vertical tank filled with liquid natural gas (LNG), the pressure head created in dip tube 8 is 1 to 2 psi. The head pressure varies with liquid density such that a heavier liquid such as argon (Density: 11.6 pounds per gallon) will generate four times the head pressure of LNG (Density: 3.5 pounds per gallon) at the same liquid height. Thus, these problems are more acute for light cryogens such as LNG. Because of the head pressure in the dip tube, the resistance to flow in the economizer line is 1 to 2 psi lower than the resistance to flow in the dip tube such that economizer line 16 will initially deliver gas to the use device thereby lowering the pressure in the tank until the pressure falls below the value set at regulator 18 at which time regulator 18 will close and liquid will be delivered via dip tube 8.
As will be appreciated, the pressure head on dip tube 8 will decrease as the dip tube is made shorter until at some length the pressure head on the dip tube will be negligible. When this occurs the opening of regulator 18 and the demand for product will result in liquid being delivered from dip tube 8 simultaneously with or in place of gas being delivered from economizer line 16. Moreover, high flow demand has the same effect as a short dip tube (i.e. the pressure head on the dip tube is negligible) because high flow causes a pressure drop in the line larger than the difference in head pressure. Under such circumstances, the pressure in tank 2 cannot be quickly or effectively be lowered.
Thus, an improved economizer system for horizontal cryogen storage tanks is desired.